Heart of Bogardan #106
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$0.71
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Card Details
Cumulative upkeep 
When a player doesn't pay this enchantment's cumulative upkeep, this enchantment deals X damage to target player or planeswalker and each creature that player or that planeswalker's controller controls, where X is twice the number of age counters on this enchantment minus 2.
Illustrated by Terese Nielsen
Rulings (1)
Apr 1, 2008
In other words, if you sacrifice Heart of Bogardan rather than pay its cumulative upkeep, X is equal to the amount of mana last spent to pay its cumulative upkeep.